Since a light emitting element is self-luminous type, it has a high level of visibility and does not require a backlight that is needed in a liquid crystal display device (LCD). This, it is suitably applied to thinner devices and not restricted in viewing angle. Therefore, a light emitting device using a light emitting element has recently been drawing attentions as a substitute display device for a CRT and an LCD.
An OLED (Organic Light Emitting Diode) is one example of light emitting elements and includes a layer containing an electroluminescent material that can obtain electroluminescence generated by applying an electric field thereto (referred to as an electroluminescent layer), an anode layer and a cathode layer. The electroluminescent layer is provided between the anode and the cathode, and further, comprises one or a plurality of layers. These layers may contain an inorganic compound in some cases. The electroluminescence in an electroluminescent layer includes a light emission when a singlet excited state returns to a ground state (fluorescence) and a light emission when a triplet excited state returns to a ground state (phosphorescence).
Next, a structure of a pixel of a conventional light emitting device and the driving method thereof will be described briefly. A pixel shown in FIG. 10 has TFTs 80 and 81, a capacitor element 82 and a light emitting element 83. A gate of the TFT 80 is connected to a scanning line 85. A source or a drain of the TFT 80 is connected to a signal line 84, and the other is connected to a gate of the TFT 81. A source of the TFT 81 is connected to a terminal 86. A drain of the TFT 81 is connected to an anode of the light emitting element 83. A cathode of the light emitting element 83 is connected to a terminal 87. The capacitor element 82 is provided for storing a voltage between the gate and the source of the TFT 81. A predetermined amount of voltages is applied from a power source to each of the terminals 86 and 87, which have a voltage difference from each other. The term voltage in this specification means an electrical difference from a ground, unless especially mentioned.
A voltage of a video signal inputted to the signal line 84 is inputted into the gate of the TFT 81 when the TFT 80 turns ON by a voltage of the scanning line 85. A gate voltage (a voltage difference between the gate and the source) of the TFT 81 is determined according to the inputted voltage of the video signal. Moreover, a drain current of the TFT 81 that flows according to the gate voltage is supplied to the light emitting element 83, and the light emitting element 83 emits light by the supplied current.